INTRODUCTION

The Lower Miocene (Lower Aragonian = Orleanian in the Neogene mammal age) lacustrine deposits of the Rubielos de Mora (Province of Teruel) in eastern Spain are known to have an abundance of fossil insects (Martínez-Delclòs et al., 1991; Peñalver and Seilacher, 1995; Montoya et al., 1996; Peñ alver, 1998a), and of articulated amphibians and plant remains (Montoya et al., 1996; Anadón et al., 2003). These deposits are located in a lacustrine basin of the Iberian Range with an asymmetrical form elongating in a NNE-SSW direction and with a surface of approximately 15 km² (Anadón et al., 1988a, 1988b, 1989).

The basin-fill sequence of the Rubielos de Mora is over 800 m thick and contains three main stratigraphic units that correspond to three evolutionary stages of basin fill (Anadón et al., 2003): (1) Lower unit of sandstones with interbedded mudstones and conglomerates, (2) Middle unit of lacustrine limestones with interbedded mudstones and sandstones, and (3) Upper unit with alluvial-deltaic, marginal lacustrine, and open lacustrine facies. The insect remains are located in the laminated grey mudstones (oil shales) and interbedded rhythmite beds of the open lacustrine facies' upper unit. The taphonomic characteristics of Rubielos de Mora indicate that this site corresponds to a Konservat-Lagerstätte as the level of preservation is relatively high. For example, soft tissues are preserved in the amphibians (e.g., eyes, gill impressions, skin, and digestive track in salamanders) and color patterns are still present in insect wings (Peñalver, 1998a, 1998b; Peñalver and Martínez-Delclòs, 2003).

One of the most important areas with fossil insect outcrops, named Río Rubielos, is located less than a kilometer to the east of the village of Rubielos de Mora (fig. 1A). To date, four outcrops have been investigated and these have been named Río Rubielos 1 (RR 1) to Río Rubielos 4 (RR 4). In 1994 an excavation was undertaken in the RR 2 outcrop that is constituted of oil shales with intercalated iron microlevels (fig. 1B). The excavation resulted in more than 820 specimens, principally insects and plants. The fossil insect association in RR 2 is constituted by the orders Diptera, Hymenoptera, Thysanoptera, Hemiptera (Heteroptera and “Homoptera”), Coleoptera, Orthoptera, and Trichoptera, in order of minor abundance (Peñalver, 1998a). Thus far, the order Hymenoptera is represented by 11 families— Ichneumonidae, Braconidae, Torymidae, Trichogrammatidae, Proctotrupidae, Diapriidae, Bethylidae, Megaspilidae, Apidae, Formicidae, and Halictidae. The Halictidae, like the Torymidae, Trichogrammatidae, and Megaspilidae, are represented only by a single specimen found in RR 2. The halictid material is the focus of the present contribution.

With over 3,460 described species, the Halictidae ranks as one of the most diverse lineages of bees, second only to the Apidae. However, the ubiquitous halictids remain one of the more taxonomically understudied groups and most surveys indicate that the family will surpass in diversity all other lineages of bees (e.g., Gonzalez and Engel, 2004). More than one-third of halictid diversity resides within the derived subfamily Halictinae, which is famous for genera such as Halictus, Lasioglossum, Sphecodes, Agapostemon, Augochlora, and Augochlorella. It is from this subfamily that the only records of fossil halictids are presently documented and despite the modern diversity, the geological history of the family is exceptionally sparse. The first description of a fossil halictid was made by Cockerell in 1906, and only eight records existed for the family by 1980. During the last quarter century the number of records has more than doubled (table 1), most coming about in the last decade, but these are still meager for a family of such diversity and are confined to a relatively few deposits (e.g., all records of Augochlorini and Caenohalictini come from a single deposit and nearly half of those of Halictini are from Florissant). Thus, the recovery of halictid remains from new deposits is of significance.

Morphological terminology follows that of Engel (2001). All metrics are of the bee as it is preserved and, thus, for some (e.g., head length) should be considered approximate given the slightly oblique position of certain tagmata in the stone (vide Preservation, infra).

SYSTEMATIC PALEONTOLOGY

DISCUSSION

Fossils of bees are uncommon and thus the recovery of any new material is of significance, particularly for a family as diverse as the Halictidae. The bees (Anthophila) are a derived group of the Apoidea that differentiated from the grade of spheciform families sometime in the late Early Cretaceous, or perhaps early mid-Cretaceous (Engel, 2001, 2004; Grimaldi and Engel, 2005). The lineage likely radiated rapidly such that derived bee lineages were already present and well represented by the Late Cretaceous (Engel, 2000b, 2004; Grimaldi and Engel, 2005). During this time bees assumed their role as the most significant pollinators of many angiosperms and indeed derived floral morphologies associated with bee pollination are well documented from the Cretaceous, particularly pollination by apine bees (e.g., Crepet and Nixon, 1998). During the Early Tertiary the bee fauna was composed of a mix of enigmatic, early forms alongside long-lived modern tribes, although the former seem to have given way during the Eocene-Oligocene transition (Engel, 2001, 2004). During the Oligocene the bee fauna seems to have become relatively modern in character, at least at the generic level. Certainly by the latest Oligocene and Miocene, as well as later epochs, the fauna was dominated by living genera or extinct genera closely allied to still extant lineages.

The family Halictidae as a whole stems from the mid-Cretaceous and is one of the more early branching lineages in bee phylogeny (Grimaldi and Engel, 2005). Fossilized nests of halictine origin are recorded from the Cenomanian (ca. 89 mya) of North America (Elliott and Nations, 1998) and from the Maastrichtian (ca. 70 mya) of Uruguay (Genise and Bown, 1996; Genise and Verde, 2000), representing some of the earliest records of bee activity. The preserved body fossils of halictids, however, are all of the derived subfamily Halictinae and from the Tertiary (rather equally distributed between the Paleogene and Neogene). However, by the earliest Eocene derived genera of Halictinae were already present among the fauna (e.g., Engel and Archibald, 2003), indicating that the diversification of halictids, particularly the separation of the subfamilial lineages, must have taken place much earlier. The tribes Augochlorini and Caenohalictini are abundant members of the Neotropical fauna, and it is, therefore, of little wonder why they are found in Tertiary amber of the Dominican Republic (table 1). Species of the Halictini, albeit still diverse, are less dominant in this region, and Lasioglossum spp., which most frequently occur here, are less likely to be entrapped by resins owing to their biology. The bias toward Halictini in compression fossils versus amber inclusions would presumably be less pronounced if more Tertiary deposits were explored in the New World, particularly South America, for compressions of Halictidae. Thus, this taphonomic bias is likely the result of poor sampling of New World deposits. Table 1 summarizes the geological records of halictid bees and their nests. The geological history of the bees (Anthophila) is discussed further by Engel (2001, 2002, 2004) and Grimaldi and Engel (2005).

Unfortunately, our understanding of the geological history of bees is “northern” biased. Little information is available on fossils from deposits in the Southern Hemisphere. For a group that was undoubtedly of Gondwanan origin (likely originating in the central, xeric regions of Gondwanaland during the early mid-Cretaceous: Engel, 2001, 2004), it will be critical to extensively document the fossil bee fauna from this large region of the globe. Thus, continued paleomelittological investigation in the field must continue before any further conclusions can be made concerning the early evolution, diversification, and historical biogeography of the bees.